Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The dose and the placement of the dose must be accurately controlled to insure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue is minimized.
Internal-source radiation therapy places capsules of radioactive material inside the patient in proximity to the tumorous tissue. Dose and placement are accurately controlled by the physical positioning of the isotope. However, internal-source radiation therapy has the disadvantages of any surgically invasive procedure, including discomfort to the patient and risk of infection.
External-source radiation therapy uses a radiation source that is external to the patient, typically either a radioisotope, such as .sup.60 Co, or a high energy x-ray source, such as a linear accelerator. The external source produces a collimated beam directed into the patient to the tumor site. External-source radiation therapy avoids some of the problems of internal-source radiation therapy, but it undesirably and necessarily irradiates a significant volume of non-tumorous or healthy tissue in the path of the radiation beam along with the tumorous tissue.
The adverse effect of irradiating healthy tissue may be reduced while maintaining a given dose of radiation in the tumorous tissue by projecting the external radiation beam into the patient at a variety of "gantry" angles with the beams converging on the tumor site. The particular volume elements of healthy tissue, along the path of the radiation beam, change, reducing the total dose to each such element of healthy tissue during the entire treatment.
The time necessary in external source radiation therapy for each therapy session can be minimized by employing a radiation source with a beam that subtends the entire cross section of a tumor, as viewed from the radiation source, as the source is rotated to different angles. In such "thick beam" systems, irradiation of healthy tissue around a tumor is limited by collimating the radiation beam to the general cross section of the tumor taken perpendicular to the axis of the radiation beam. Many systems collimate radiation by using a circumferential collimator that employs multiple sliding shutters which, piecewise, may generate a radiopaque mask of arbitrary outline. When using such a collimator with a thick beam, irradiation may be accomplished at a limited number of angles and therefore therapy sessions can be performed quickly.
Although circumferential collimation limits the irradiation of healthy tissue outside the cross section of the irradiated tumor, it does not allow a therapist to regulate the dose within the tumor or to accurately control the dose received by irregular or convex tumors. Such circumferential collimation is best when a tumor is removed from radiation sensitive organs and the tumor may be approximated by a regular convex shape such as a cylinder or sphere.
When a radiation sensitive organ is near a tumor or the tumorous volume is of an irregular shape, the ability to regulate the fluence directed toward different parts of the tumor becomes very important. Ideal radiation therapy under these conditions requires that the intensity of each adjacent section of the radiation beam (along both a beam width and a beam thickness perpendicular to the width) be separately controllable.
A radiotherapy machine that regulates the fluence of a beam in such a manner is presented in co-pending U.S. patent application Ser. No. 07/854,521, filed Mar. 19, 1992 by Stuart Swerdloff et al. That application discloses a radiotherapy machine employing a fan beam for irradiating a sequence of "slices" of a tumor. A compensator disclosed therein varies the fluence of adjacent rays within the fan beam width.
The compensator includes a number of radiation attenuating leaves disposed along the fan beam width that move into the fan beam in a closed state, each leaf thus occluding one ray of the beam, and move out of the fan beam in an open state to allow unobstructed passage of the associated rays. A timer controls the ratio of the time during which each leaf is in the closed state to the time during which each leaf is in the open state thereby controlling the average intensity of each ray of the beam width independent of the other rays.
The compensator's ability to vary the intensity of individual rays within the beam width and among tumor slices, as opposed to simply collimating the edges of the beam, allows advanced techniques of therapy planning to be employed. In such techniques a fluence profile across the width of the beam is varied for each angle about the tumor to accurately control the radiation dose delivered to and around the tumor.
In this technique, a "pixel" of exposure is defined by the number of controllable rays in the fan beam and hence the width of each ray and the fan beam thickness. Smaller pixels provide better dose placement.
When a thin fan beam is used, smaller pixels are obtained, but more slices are needed to treat a tumor and hence more gantry rotations are required. This prolongs the treatment time because the speed of gantry rotation is limited by its fluence which is essentially constant regardless of fan beam width. If gantry rotation speed is increased, the radiation absorbed by the tumor slice is decreased.
In addition, gantry rotation speed is also limited by the maximum switching speed of the compensator leaves. To provide the desired duty cycle for each beam ray, the compensator leaves must be moved once in and out of the beam at each gantry angle and the next gantry angle cannot be assumed until that movement is complete.
The tradeoff between treatment time and pixels size is illustrated by two polar design approaches for a compensator based system. One design approach would use a thick beam using a circumferential collimator. This design would provide fast therapy but large pixels and thus coarse fluence regulation. The second design approach would use a thin beam and require more time per therapy session producing finer fluence regulation.